1. Field of the Invention
The present invention relates to radio receivers and radio transmitters that compensate for the influence of frequency drifts in oscillation circuits that generate carriers, local oscillation signals, and other signals.
2. Description of the Related Art
In radio communications, such as a keyless entry system that requires a short time to transmit and receive a control code having a short length, a radio transmitter transmits a radio signal on which information, such as an identification code and a control code that are allocated for each object, is superimposed, and a radio receiver receives the radio signal and demodulates the codes. Then, in accordance with the demodulated codes, a controller authenticates the object and locks/unlocks a door.
In such radio communications, the radio transmitter must include an oscillation circuit that oscillates a high-frequency signal, which functions as a carrier, and the radio receiver must include a local oscillation circuit that performs frequency conversion. In addition, the radio signal must be transmitted and received in a state in which each of the frequency of the carrier of the radio signal from the radio transmitter and the frequency of the local oscillation circuit of the radio receiver is set to a predetermined value.
However, frequency drift in which an oscillation frequency varies in accordance with a temperature change, a time-lapse change in a circuit constant, and a time-lapse change in an oscillator often occurs in an oscillation circuit for transmitting or receiving radio signals. If a carrier frequency of a radio signal transmitted from the radio transmitter or an oscillation frequency of the local oscillation circuit of the radio receiver deviates from a predetermined value due to such a frequency drift, the frequency of an intermediate-frequency signal acquired by mixing the radio signal with the local oscillation signal in the receiver deviates from a predetermined value. Thus, there is a problem in that if the deviation of the intermediate-frequency signal increases, transmission and reception of the radio signal cannot be performed.
When a wide frequency band is used for a radio signal, the frequency deviation problem may be ignored even if a certain amount of frequency drift occurs. In contrast, signal noise that is produced when a radio signal is received increases in proportion as the frequency bandwidth for the radio signal increases. Thus, a problem occurs in that the reception sensitivity is reduced and communication reliability, such as a BER (Bit Error Rate), decreases. Thus, the communication reliability must be improved by using a narrow frequency band for the radio signal.
Normally, when transmission and reception of a radio signal must be performed under the above-described conditions in which a narrow frequency band is used for the radio signal, a method is used in which the influence of a frequency drift is compensated for without degrading the communication reliability, by providing an oscillator having a temperature compensation function, such as a TCXO, as a reference oscillation signal source.
According to this method, since the frequency drift is compensated for by the oscillator having the temperature compensation function, even if the frequency bandwidth for a radio signal is narrow, transmission and reception of the radio signal is reliably performed.
However, when the oscillator having the temperature compensation function, such as a TCXO, is used, the unit cost of such an oscillator is substantially greater than an oscillator which does not have a temperature compensation function. Thus, there is a problem in that the overall cost of the entire radio communication system is increased.
Thus, radio communication systems that compensate for the influence of a frequency drift without using an oscillator having the temperature compensation function, such as a TCXO, are disclosed in Japanese Unexamined Patent Application Publication No. 08-139773 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 11-348732 (Patent Document 2).
In Patent Document 1, prior to transmission of information to be communicated, a radio transmitter performs FSK modulation on a signal including a bit synchronization signal and transmits the FSK-modulated signal (a preamble method), and a radio receiver receives the signal and controls a local oscillation circuit to have a frequency at which the level of the reception signal is maximized. The radio system described in Patent Document 1 ensures establishment of communication by synchronizing, in advance, the frequency of the local oscillation circuit of the radio receiver with a frequency corresponding to a carrier frequency.
In addition, in Patent Document 2, a radio receiver sweeps an oscillation frequency of a local oscillation circuit, monitors the strength of a reception signal (RSSI: Radio Signal Strength Indicator), and stops sweeping the oscillation frequency when the strength of the reception signal reaches a high level. Accordingly, the oscillation frequency of the local oscillation circuit is adjusted. In addition, when the strength of the reception signal reaches a high level, the bandwidth of an IF signal filter is switched from a wide bandwidth to a narrow bandwidth. Accordingly, the BER is reduced, and the reception sensitivity is improved.
In the communication systems described in Patent Documents 1 and 2, even if the frequency bandwidth for a radio signal is narrowed, the influence of a frequency drift is compensated for. In contrast, however, the communication system described in Patent Document 1 requires a circuit for switching between a bit synchronization signal and a radio signal. Thus, this communication system requires a large, complicated circuit. In addition, the communication system described in Patent Document 2 requires an RSSI monitoring circuit, a plurality of filters, a filter switching circuit, and other additional circuits. Thus, this communication system also requires a large complicated circuit, and has a problem of degradation of communication reliability. As described above, in the related art, the number of component parts increases, and the area required on an IC increases. Thus, a new problem arises in that the cost of the entire radio communication system is substantially increased.
As described above, in the related art, the influence of a frequency drift can be corrected while ensuring communication reliability by narrowing the frequency bandwidth for a radio signal. However, at the same time, such a circuit is complicated, the number of component parts increases, and the area required on an IC increases. Thus, a problem arises in that the cost of the entire communication system is increased.